Nickel-chromium-iron alloy

ABSTRACT

The invention concerns an austenitic aluminum-alloyed steel uniquely adapted as material for the exhaust cleaning system of a combustion or explosion motor or similar high temperature purpose. The alloy has the following composition expressed in weight percentages:   D R A W I N G

[ 1 Aug. 19, 1975 NIC KEL-CHROMlUM-IRON ALLOY [75] Inventors: Udo Walter Vogt; Sven G. \Naher,

both of Sandviken, Sweden [73] Assignee: Sandvik Aktiebolag, Sandviken,

Sweden [22] Filed: Feb. 15, 1974 [21] Appl. No.: 443,097

[30] Foreign Application Priority Data Feb. 20, 1973 Sweden 7302336 [52] [1.8. CI. 75/124; 75/128 T [51] Int. Cl. C22C 38/06; C22C 38/50 [58] Field of Search 75/124, 128 T, 128 A, 122

[56] References Cited UNITED STATES PATENTS 264L540 6/[953 Mohling et al 75/128 A X 3,300,347 H1967 Kasza et al 75/128 A X Primary litanxiner-C. Lovell Assislam E.\'amiuerArthur J Steiner Attorney, Agent, or Firm-Pierce, Scheffler & Parker [57] ABSTRACT The invention concerns an austenitic aluminumalloyed steel uniquely adapted as material for the exhaust cleaning system of a combustion or explosion motor or similar high temperature purpose. The alloy has the following composition expressed in weight percentages:

carbon 0102 0. l 5 chromium l8 24 nickel 22 3l aluminum 1.! 2.5 titanium 0.1 08 silicon 0.3 1.2 manganese 013 2.0 iron balance besides normally present impurities.

12 Claims, 1 Drawing Figure NICKEL-CHROMIUM-IRON ALLOY The present invention relates to an austenitic aluminum-alloyed steel, having high contents of chromium and nickel, which is intended essentially to be used as material for exhaust cleaners of combustion or explosion motors. The steel alloy has been developed in consideration of the very special demands which are made upon materials for thermal and catalytic exhaust reactors and similar high temperature purposes.

Material properties of great importance in this connection are: high resistance to oxidation, particularly at rapid cyclic temperature variations; high creep strength in order to resist thermal as well as mechanical strain; and good structure stability (low tendency to embrittlement). Besides these, there are manipulative properties including good workability and weldability, which latter are required in order to make possible manufacturing by conventional means.

According to the present invention it has now been found possible to obtain a material which fulfills the mentioned requirements of properties or qualities by means of a well-balanced or optimum addition of alloy ing elements. The alloy according to the invention consists essentially of, in by weight, 0.02 0.15 C, 18 24 %Cr 22- 31% Ni, 1.1-2.5 %Al,0.1-0.8 Ti, 0.3 1.2 Si, 0.3 2.0 Mn and the remainder Fe besides normally present impurities.

When the steel is used in exhaust cleaners the carbon content should be chosen in regard to two essentially different factors depending upon the operating conditions of the reactor. When the cleaner is used at low operating temperatures (i.e., less than about 850 C. the C content of the steel should be low and preferably at the most 0.06 wt. in order to avoid precipitation of chromium carbide in the grain boundaries. Such precipitation brings risks of intercrystalline corrosion at shut-off of the reactor. on the other hand, in a reactor essentially intended for long time operating at higher temperatures (i.e., at above about 850 C.), a higher C-content is preferred in regard to creep strength as well as resistance to oxidation. A suitable carbon content has shown to be 0.08 0.13

Ti is added for the main purpose of improving the creep strength of the steel and to bind the carbon as for example in welding operations. It has been found that a suitable content of titanium is 0.2 0.6 wt.

Of the main alloying elements Ni and Cr, nickel contributes essentially to making a stable austenitic structure and thus a good creep strength and toughness, while chromium essentially gives the alloy a high resistance to oxidation. Contents of Ni lower than about 24 weight r. is generally not used because of the risks of formation of embrittling sigma-phase. lt is per se known that an increasing addition of Ni (in contents of about 60 70) has a favorable influence upon the re sistance to oxidation in steel alloys having a high content of Cr (above about "/(J. On the other hand, it is also known that contents of Ni, above what is necessary to give a stable austenitie structure, impair the creep strength. It has been found that a suitable Ni con tent of the alloy according to the invention is 26 30 wt. 72 in consequence of the above mentioned points. The content of Cr must be well balanced with regard to the fact that an increasing content gives unfavorable sigma-phase formation, which means a lower creep strength and toughness. An optimum chromium content, in view of resistance to oxidation as well as sigma phase formation, has been shown to be at the lowest 19% and preferablly at the most 22% Cr (by weight).

Temperature variations accelerate the rate of oxidation. particularly as the oxide scales off because of the thermal stresses which are formed between the oxide and the metal beneath. The resistance of the oxide to thermal fatigue can be improved by additions of rare earth metals such as Ce, Y and La. These additions are active, however, up to a certain temperature limit, which in austenitic materials is regarded to be at 900 C. Furthermore, these additions give an impaired hot-workability and weldability of the alloy.

The adherence of the oxide can also be improved by additions of Si and as examples may be mentioned older, known steels with nominal compositions of 25 Cr, 20 Ni, 2 Si, remainder Fe respectively 20 Cr, 38 Ni, 2 Si, remainder Fe. Because of their high Si contents (about 2 Si) these kinds of steels have a strong tendency to embrittlement in the precipitation of sigma phase at long time use 10 10 hours) in the temperature interval 600 850 C. It is also known that the last mentioned steel has a lower creep strength compared to that of the corresponding Si-free alloy. Furthermore, it is known that these steels because of their high Si contents are sensitive to hot cracking during welding. 1n the alloy according to the invention the content of Si is caarfully balanced in order to reach an improved adherence of the oxide without causing the mentioned disadvantages as embrittlement, creep strength and so on. A suitable Si content has shown to be 0.5 1.0 wt.

The content of manganese should be the normal for this type of alloy. it is usually 0.4 1.0 wt.

Particularly characteristic of the alloy according to the invention is the relatively high, and carefully adjusted, addition of Al in combination with a well balanced Ni content. The addition of Al gives a certain inner oxidation which improves the adherence of the oxide to the metal. This favorable effect appears at A1 contents of about 1.5 wt. Higher contents of Al than necessary to reach the above-mentioned favorable effect should be avoided, however, on behalf of the structure stability of an article formed from the steel. The optimum addition of A1 in view of oxidation effects is about 1.2 2.1 (usually 1.5%).

The mentioned technique with addition of Al had earlier been applied in a commercial Ni-alloy with a nominal composition of 23 Cr, 60 Ni, 1.3 A1. This alloy contained, however, an unnecessarily high content of Ni from a technicabeconomieal point of view. It is known, as has been mentioned earlier, that Ni has a favorable influence upon the resistance to oxidation in Fe-Cr-Ni-alloys, but now it has been found by our tests that the content of Ni in the Al-alloyed Fe-Cr-Ni-steels can be lowered down to 25 without seriously impairlowered ing the alloy's resistance to oxidation.

The following examples from oxidation tests illustrate the foregoing statements:

invention.

2) Commercial Ni-alloy 3) Test grade.

4) Commercial FeCr-Ni-alloy Alloy 800 (In the alloys, the remainder is Fe besides small alloying additions and normal impurities.)

The results show that the alloy according to the invention has a resistance to oxidation which is equal to the resistance of the essentially more expensive Ni-alloy. In comparison with the tested Al-free Fe-Cr-Ni-alloy (No. 4) a considerable improvement was realized. The following examples give certain results from comparative creep strength tests:

C rcep strength testing Alloy Time to rupture (h) at 900C at the strain (kp/mm) l 20 Cr 30 Ni 300 700 2 lo Cr 78 Ni 300 700 3 20 Cr 30 Ni 3 Al 200 600 4 20 Cr 29 Ni L5 Al 500 480() Remarks: l Commercial Fe-C'r-NLalloy Alloy 800".

2) Commerical Ni-alloy 3) Test grade. 4) Alloy according to the invention.

"Alloy 600.

It will be seen that the alloy according to the invention has an essentially improved creep strength compared to that of the corresponding Al-free alloy as well as that of an alloy having too high contents ofAl (3 The results further emphasize the importance of a well-balanced and carefully determined content of Al (alloy 3 respectively 4). Such a well defined content has been found to be 1.3 1.7 wt. Al.

As has been mentioned earlier the alloy according to the invention is uniquely useful as material of construction for exhaust cleaners. The material is usually supplied as strip steel in annealed-pickled, bright annealed, or cold-rolled condition, and in thicknesses between 0.2 4 mm and widths between l-400 mm, but can also be produced as wire or in the form of welded tub- As had been mentioned earlier, the alloy is essentially intended for use in exhaust cleaning systems such as thermal and/or catalytic reactors, sensors (being details in carburator controlling systems) and appertaining parts. The invention as so applied is illustrated in the single FIGURE of drawing showing a thermal reactor of conventional form. The inner part of the reactor consists of a combustion chamber 1, a burning tube 2 and inlet pipes 3 for the exhaust gas. These parts, and other details exposed to high temperatures, are made of the alloy according to the invention. The FIGURE shows also the insulation 4 of the chamber and its outer case 5.

In the operation, exhaust gas mixed with air is let into the reactor through the inlet pipes 3. The gas is burned in the burning tube 3, which is perforated for supply of further combustion air, coming from an inlet (not shown) and for removal of the burned exhaust gases through an outlet (also not shown).

We claim:

1. Nickel-ehromium-iron alloy having excellent resistance to oxidation at rapid cyclic temperature variations, high creep strength and good structure stability, said alloy having a composition consisting essentially of in by weight: 0.02 0.l5 Cr; 18 24 Cr; 22 31 %Ni; 1.1 -2.5%Al;0.l 0.8 %Ti',0.3-1.2%Si; 0.3 2.0 Mn, and the remainder Fe besides normally present impurities.

2. Alloy according to claim 1, wherein the C-content is at the most 0.06

3. Alloy according to claim 1, wherein the C-content is 0.08 0.13

4. Alloy according to claim 1, wherein the Ti content is 0.2 0.6

5. Alloy according to claim 1, wherein the Ni content is at least 24%.

6. Alloy according to claim 5, in which the Ni content is 26 30%.

7. Alloy according to claim 1, wherein the Cr content is at least 19% and preferably at the most 22%.

8. Alloy according to claim 1, wherein the Si content is 0.5 1.0

9. Alloy according to claim 1, wherein the Mn content is 0.4 1.0

10. Alloy according to claim 1, wherein the Al content is 1.2 2.1%.

11. Alloy according to claim 1, wherein the Al content is l.3 l.7

12. An exhaust cleaner of a combustion or explosion motor the parts of which cleaner come into contact with high-temperature gases are formed from a nickelchromium-iron alloy having a composition consisting essentially of, in weight percentages:

C 0.02 0. [5 Cr l 8 24 Al I l 2.5

Ti 0. I 0.8

Si 0.3 l .2 Mn 0.3 2.0 Fe balance. besides normally present impurities.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,900,315 DATED August 19,1975

INVENTOR(S) Udo Walter Vogt,et a].

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown beinw:

Claim 1 line 5 "Cr" (first occurrence) should read:

Signed and Scaled this th SEAL! foul D y of Novemberl 975 RUTH C. MASON C. MARSHALL DANN M /17 (nmmimuner nfPamm and Trademark: 

1. NICKEL-CHROMIUM-IRON ALLOY HAVING EXCELLENT RESISTANCE TO OXIDATION AT RAPID CYCLIC TEMPERATURE VARIATIONS, HIGH CREEP STRENGTH AND GOOD STRUCTURE STABILITY, SAID ALLOY HAVING A COMPOSITION CONSISTING ESSENTIALLY OF IN % BY WEIGHT: 0.02-0.15 % CR, 18 - 24 % CR, 22-31 % NI, 1.1 -I.5 % AI, 0.1 -0.8 % TI, 0.3 - 1.2 % SI, 0.3 - 2.0 % MN, AND THE REMAINDER FE BESIDES NORMALLY PRESENTS IMPURITIES.
 2. Alloy according to claim 1, wherein the C-content is at the most 0.06 %.
 3. Alloy according to claim 1, wherein the C-content is 0.08 -0.13 %.
 4. Alloy according to claim 1, wherein the Ti content is 0.2 -0.6 %.
 5. Alloy according to claim 1, wherein the Ni content is at least 24%.
 6. Alloy according to claim 5, in which the Ni content is 26 -30%.
 7. Alloy according to claim 1, wherein the Cr content is at least 19% and preferably at the most 22%.
 8. Alloy according to claim 1, wherein the Si content is 0.5 -1.0 %.
 9. Alloy according to claim 1, wherein the Mn content is 0.4 -1.0 %.
 10. Alloy according to claim 1, wherein the Al content is 1.2 -2.1 %.
 11. Alloy according to claim 1, wherein the Al content is 1.3 -1.7 %.
 12. An exhaust cleaner of a combustion or explosion motor the parts of which cleaner come into contact with high-temperature gases are formed from a nickel-chromium-iron alloy having a composition consisting essentially of, in weight percentages: 